1. Field of the Invention
The present invention relates to a method of forming single-crystal semiconductor films. More particularly, the present invention relates to a method of forming single-crystal semiconductor films on an insulator layer via a seed hole which goes through the insulator layer which is formed on a single-crystal semiconductor substrate.
2. Description of the Related Art
To enhance the performance of a semiconductor device, attempts have been made to manufacture a laminated semiconductor device by a method in which circuit elements are three-dimensionally laminated into a multilayered form. In such a method, a step is needed in which only non-single-crystal semiconductor layers are heated and melted, while a semiconductor substrate temperature is being kept at a relatively low temperature of 800.degree. C. or lower, by irradiating an energy beam which is finely condensed onto a non-single-crystal semiconductor layer deposited on an insulator layer on a semiconductor substrate, the non-single-crystal semiconductor layer being transformed into a single crystal. The substrate temperature is kept at 800.degree. C. or lower to prevent impurities in impurity portions of circuit elements formed in the substrate from being diffused to other portions.
Referring to FIGS. 5 and 6, a conventional method of forming single-crystal semiconductor films will now be explained. FIG. 6 is a cross-sectional view taken along line I'I in FIG. 5. A silicon oxide film 2 having a thickness of approximately 1 .mu.m is formed on a silicon substrate 1. A seed hole 3 having a diameter of approximately 3 .mu.m is so provided that it goes through the silicon oxide film 2. A polycrystalline silicon film 4 having a thickness of approximately 0.5 .mu.m is deposited by a chemical vapor deposition (CVD) method on the silicon oxide film 2. A plurality of silicon nitride film patterns 5a and 5b, which are deposited to a thickness of approximately 500.ANG. by the CVD process and patterned, are provided on the polycrystalline silicon film 4. The nitride film patterns 5a having a quadrilateral shape, one side of which has a length of approximately 4 .mu.m, are provided above the seed hole 3. The nitride film was patterned into stripes with lines and spaces of 5 .mu.m and 10 .mu.m, respectively. A single-crystal silicon film is formed by melting the polycrystalline silicon film 4 in a direction along the nitride film patterns 5b by energy beam irradiation starting from the seed hole 3.
The step in which the polycrystalline film 4 is melted and transformed into a single crystal is shown in FIG. 7. The silicon substrate 1 is heated to approximately 450.degree. C. by an unillustrated resistor heater. A laser beam 6, a cross section of which is condensed into the shape of a circle 6a having a diameter of approximately 100 .mu.m, is scanned along the nitride film patterns 5b at a speed of 25 cm/s in a direction indicated by the arrow L, starting from the silicon nitride film pattern 5a formed above the seed hole 3. This laser beam 6 can be obtained by, for example, a continuous oscillation type argon laser having an output of 12 W.
The polycrystalline silicon film 4 melts in a portion 4a irradiated by the laser beam 6, and the silicon oxide film 2 is softened in a portion 2a directly below the portion 4a. The melted silicon solidifies again after the laser beam 6 has passed by it. As a result, a single-crystal silicon film 4b having a crystal axis transferred from the silicon substrate 1 through the seed hole 3 is epitaxially grown.
The nitride film patterns 5b shown in FIG. 5 function as films for preventing reflection of the laser beam 6 and control the distribution of temperatures in a direction traversing the direction in which single crystals are grown so that the single crystals are grown long from the seed hole 3. The silicon nitride film patterns 5a, having a quadrilateral shape, provided above the seed hole 3, are provided to prevent the laser beam 6 from reflecting and increase heat absorption, so that escape of heat, due to the silicon filled in the seed hole 3 having heat conductivity larger than that of the oxide film 2, is compensated for.
Such an above-described mechanism according to the prior art for forming silicon single-crystal films is described in detail in "Extended Abstract of the 18th (1986) International Conference on Solid State Devices and Materials, Tokyo, 1986, pp.565-568".
Single-crystal semiconductor films according to the prior art have been formed as described above. However, there is a problem in that, the crystal axis of the single-crystal silicon film 4b obtained in the above-described way, it is rotated continuously in proportion to the distance from the seed hole 3.
FIG. 8 is a graph, in which the angle between the crystal axis of the single-crystal silicon film 4b and the crystal axis of the silicon substrate 1 measured by electron channeling pattern (ECP) are shown with respect to the distance from the seed hole 3. The horizontal axis shows the distance (.mu.m) from a seed and the vertical axis shows the crystal axis rotational angle (in degrees). The curve A indicated by white circular marks shows the rotational angle of the crystal axis of the single-crystal silicon film 4b obtained by the prior art. It can be seen from this curve A that the crystal axis is rotated as much as approximately 30 degrees at a distance of 300 .mu.m from the seed hole 3. Such a phenomenon concerning the rotation of the crystal axis can be explained as described below.
FIG. 9 is a schematic sectional view in which the two-dimensional temperature distribution in the vicinity of the interface between, as shown in FIG. 7, the portion 4a where silicon is melted and the single-crystal silicon film 4b, indicated by an isothermal line determined by simulation. In this figure, the silicon melted band 4a progresses from left to right. An isothermal line MP indicated by a broken line in FIG. 9 indicates a temperature 1,414.degree. C., which is the melting point of silicon. The solid-liquid interface between the silicon melted band 4a and the single-crystal silicon film 4b exists along this isothermal line MP. AS can be seen from the isothermal line MP, the solid-liquid interface is inclined backwards when it is considered that the silicon melted band 4a progresses in a forward direction. That is, in the vicinity of the solid-liquid interface, the temperature of the upper layer portion of the single-crystal silicon film 4b is higher than that of the lower layer portion thereof.
Therefore, the crystal lattices in the upper layer portion of the single-crystal silicon film 4b are more extended due to heat expansion than the crystal lattices in the lower layer portion. In such a condition, the silicon atoms in the silicon melted band 4a are epitaxially solidified again on the crystal lattices bent on the solid-liquid interface, and fixed to the silicon oxide film 2 of the lower layer in that condition. In this way, the crystal axis of the single-crystal silicon film 4b is gradually rotated as the growth thereof progresses.